That stands for "designated hitter." In the National League, all nine guys who come onto the diamond take their turn at the plate. In the American League, pitchers don't bat because a guy known as the designated hitter does it for them. (Unless they're playing in a National League park which is how Felix Hernandez hit a grand salami off Johan Santana in 2008.) I'm highly in favor of the DH because 1) most pitchers even in the NL don't hit very well so what's the point and 2) pitchers are often the most expensive players on the team and who wants to take a chance having one out of the rotation because he got cleated by a pissed-off opponent while running the bases? Edgar Martinez DHed for the Mariners, did a spectacular job and, it is hoped, will become the first DH to go into the Hall of Fame.